This project will focus on defining the role of the cell cycle inhibitor, p21, in normal immune and autoimmune responses. The cell cycle plays a critical role in determining the fate and differentiation state of cells. It is therefore highly regulated by complexes of proteins such as cyclin, cyclin-dependent kinases (CDKs) and CDK inhibitors (CDKIs)7 which in turn are regulated by external stimuli through a number of intracellular signaling pathways. We found that high levels of certain CDKIs, including p21, p18 and p27 are present in activated/memory (CD44hl) phenotype CD4+ T cells in lupus-prone male BXSB mice. Such T cells are commonly increased in lupus and we postulated that repeated stimulation of self-antigen-reactive T cells might lead to a state similar to "replicative senescence", where T cells are no longer cycling, but are resistant to apoptosis, accumulate and transcribe autoimmune-promoting pro-inflammatory cytokines. To test this possibility, we generated and examined lupus-prone mice deficient in p21 and have performed preliminary studies of p27 knockout autoimmune mice. Male BXSB mice lacking p21 exhibited marked reduction in disease associated with enhanced apoptosis of T and B lymphocytes and significantly decreased accumulation of activated/memory CD4+ T cells. Initial studies of p27- deficient male BXSB mice also revealed reduced mortality. To further examine the role of CDKIs in autoimmunity and normal immune response four aims are proposed. The first will determine the effect of p21 deficiency on another genetically distinct lupus-prone strain, NZB.NZW-Lbw5, an interval-specific congenic line derived from NZB and NZW mice. The second aim will determine whether expression of p21 in T cells is critical for the development of autoimmunity. The third aim will examine the effect of p21 deficiency on immune responses to a foreign antigen and LCMV infection. The final aim will determine the role of Fas-induced apoptosis in the disease reduction observed in BXSB-p21"7" mice. These studies should yield important new insights into the significance and role of p21 in systemic autoimmunity and in host response to foreign antigens.